As discovered by Jackfrags (full video below via VG247) here's a GIF from Reddit helpfully showing it in action:

Best of all this isn't a comedy shooter for the sake of it, it's a 2.7mm Kolibri Car Pistol - a real gun made by an Austrian watchmaker called Franz Pfannl in 1914.

Here's a picture of it next to a normal sized bullet:

Clearly Battlefield 1's upped its stats in the game as the gun was reportedly very inaccurate, hard to load (because it's tiny) and had 0.2g bullets that were only capable of penetrating 10-40mm. As reddit suggests, it has to be a troll weapon.